1. Field of the Invention
The present invention relates to a voltage comparator having hysteresis characteristics, and more specifically, to a voltage comparator having hysteresis characteristics, in which a reference voltage changing section composed of a small number of elements is added so that a voltage comparator safe from input noise can be designed and the magnitude of a hysteresis voltage can be accurately set and simply changed by varying the value of a resistance.
2. Description of the Related Art
In general, a voltage comparator compares an input voltage with a reference voltage and amplifies the difference therebetween so as to output a high-level or low-level signal. Since a conventional voltage comparator does not have a function of compensating for noise with respect to an output, a separate analog or digital compensating circuit is added thereto.
As a circuit added to the voltage comparator in order to solve the noise problem, there is provided a Schmitt trigger circuit having hysteresis characteristics. However, the Schmitt trigger circuit is sensitive to process variation when determining a positive threshold voltage Vth+ and negative threshold voltage Vth−.
Recently, a voltage comparator itself is designed to have hysteresis characteristics.
After comparing an input voltage with a reference voltage, the voltage comparator having hysteresis characteristics outputs a high-level or low-level signal if the input voltage is larger than the reference voltage. That is, the voltage comparator is used in comparing an input signal.
The voltage comparator having hysteresis characteristics has two points where the output changes, that is, an upper reference voltage and lower reference voltage.
In order to implement a voltage comparator which is safe from noise, the voltage comparator should be designed to have hysteresis characteristics. Accordingly, if the hysteresis characteristics are varied in accordance with a process, an error is generated in the voltage comparator itself, thereby deteriorating the reliability of the overall semiconductor.
FIG. 1 is a circuit diagram showing a voltage comparator 100 according to the related art. As shown in FIG. 1, the conventional voltage comparator 100 is composed of first and second transistors 101 and 102 which are driven by the input voltage Vin, third and fourth transistors 103 and 104 which are driven by the reference voltage Vref, a voltage drop unit 105 which is connected to the second and third transistors 102 and 103 and a ground terminal so as to generate a voltage drop, a fifth transistor 106 which is connected to the second transistor 102 and a ground terminal so as to be driven by the voltage generated by the voltage drop unit 105, a sixth transistor 107 which is connected to the fifth transistor 106 and a ground terminal so as to be driven by a power supply voltage, and a bias current section 108 which is connected to the first, fourth, fifth, and sixth transistors 101, 104, 106, and 107 and a common terminal of the second and third transistors 102 and 103 so as to maintain a constant amount of current.
The first to fourth transistors 101 to 104 are composed of PNP transistors, and the fifth and sixth transistors 106 and 107 are composed of NPN transistors.
The voltage drop unit 105 is composed of seventh and eighth transistors 105a and 105b connected in a current mirror relationship, and the seventh and eighth transistors 105a and 105b are composed of NPN transistors.
When the voltage comparator 100 having such a construction compares the input voltage Vin and the reference voltage Vref so as to output a high-level or low-level signal, the process is explained in FIG. 2, and will be described in detail as follows.
FIG. 2 is a diagram showing an operational process of the conventional voltage comparator with an output signal according thereto. FIG. 2A shows an input voltage Vin and reference voltage Vref of the voltage comparator in accordance with time. FIG. 2B shows an output signal Vout of the voltage comparator in accordance with time.
First, when the input voltage Vin is smaller than the reference voltage Vref, that is, in an interval A, the second transistor 102 is turned on and the third transistor 103 is turned off, because of a characteristic of PNP transistors. Then, the bias current I1a of the voltage comparator 100 flows into I2a. Accordingly, a voltage drop is generated in the voltage drop unit 105, so that a constant voltage is applied to the emitter of the fifth transistor 106. At this time, since the voltage is larger than the threshold voltage of the fifth transistor 106, the fifth transistor 106 is turned on.
Therefore, the emitter of the sixth transistor 107 receives a collector-emitter voltage of the fifth transistor 106. Since the voltage, which is about 0.1 V, is less than the threshold voltage of the sixth transistor 107, the sixth transistor 107 is turned off.
Then, the output terminal of the voltage comparator 100 receives a power supply voltage VDD, so that a high-level signal is accordingly output, as shown in FIG. 2B.
When the input voltage Vin is larger than the reference voltage Vref, that is, in an interval B, the second transistor 102 is turned off and the third transistor 103 is turned on, because of a characteristic of PNP transistors. Then, the bias current I1a of the voltage comparator 100 flows into I3a. In this case, a voltage drop is not generated in the voltage drop unit 105, and an emitter voltage which can turn on the fifth transistor 106 is not applied. Accordingly, the fifth transistor 106 is turned off.
Therefore, the emitter of the sixth transistor 107 receives a power supply voltage VDD of the voltage comparator 100. Then, the sixth transistor 107 is turned on, so that a low-level signal is output to the output terminal of the voltage comparator 100, as shown in FIG. 2B.
FIG. 3 is a diagram showing an output signal in accordance with the input noise in the conventional voltage comparator. FIG. 3A shows an input voltage Vin and reference voltage Vref when noise is generated in an interval where the input voltage Vin is larger than the reference voltage Vref. FIG. 3B shows an output signal when the noise is generated.
In a stable voltage comparator, the output of the voltage comparator is maintained at low level, even though noise C is generated in the input voltage Vin at an interval where the input voltage Vin is larger than the reference voltage Vref. However, the above-described conventional voltage comparator responds to the noise C so as to output a high-level signal, as shown in FIG. 3.